The goal of this project is to build a new, robust, and compact source for Coherent anti-Stokes Raman Spectroscopy (CARS) microscopy based on fiber lasers. CARS is a potentially powerful technique for microscopy because the molecular identity of cellular components (e.g. lipids, proteins, or DNA) can be imaged without the need for fluorescence labeling. Optical sectioning is intrinsic to the CARS process because of the nonlinear dependence on pulse intensity. In addition, near-infrared wavelengths can be used for increased tissue penetration. Therefore, CARS microscopy has the potential to be an important biomedical and clinical tool. However, the laser sources required are typically bulkly, expensive and hard to handle, and as a result CARS microscopy has so far found limited application. We propose to develop a technique based on fiber lasers: Fiber Laser EXcitation CARS (FLEX- CARS), which uses a single fiber laser as the source of pump and Stokes pulses. A nonlinear optical fiber (photonic crystal fiber) will be used to shift the wavelength of the Stokes pulses. As a result, pump, probe, and Stokes are automatically synchronized in time. We will develop FLEX- CARS based on a fiber laser at 1030 nm. The frequency resolution of FLEX-CARS will be significantly improved by a novel pulse chirping method that utilizes the full pulse power of pump and Stokes pulses. The wavelength of the Stokes beam can be tuned or switched rapidly on the microsecond time scale, allowing new methods to probe multiple vibrational frequencies. Signal quality will be enhanced by rejection of nonresonant background components through the implementation of polarization and time-delay methods to select the vibrationally resonant signal. The unique capability for fast wavelength switching in the proposed system also makes frequency modulated (FM) CARS straightforward, which helps further rejection of non-resonant background. FLEX-CARS will be applied first to standard samples and then to brain tissue to demonstrate the potential of the method as a robust system for CARS microsocpy.